Winter Tetany, Milk Fever, and Pregnancy Toxaemia – Downer Cows

Introduction
In years of drought—and conversely, in years of excessive moisture—winter feed quality for beef herds can be significantly compromised. Across Western Canada, producers have reported an increase in “down cows”, both prior to and shortly after calving.
Often, these cases are not caused by one single factor but by a combination of metabolic and nutritional issues, including:
  • Prolonged negative energy balance
  • Hypomagnesemia (low magnesium)
  • Improper calcium-to-phosphorus ratios
  • Imbalance in dietary cation–anion balance (DCAB)
Drought years often force producers to feed more greenfeed and straw, both low in calcium and potentially contributing to milk fever. In wet years, moldy feeds and nutrient dilution may compromise energy, vitamins, and mineral levels—sometimes increasing potassium or decreasing calcium.
This section summarizes the main conditions associated with down cows and the underlying nutritional and metabolic causes.
 
Metabolic Disorders Leading to Down Cows
Milk Fever (Acute Hypocalcemia)
Milk fever occurs when blood calcium levels drop below 1.6 mmol/L.
Symptoms
  • Muscular weakness
  • Low body temperature
  • Increased heart rate
  • Sternal recumbency
  • Loss of consciousness
Cause and Treatment
Milk fever results from the cow’s reduced ability to mobilize bone calcium at calving. Treatment involves IV or subcutaneous calcium gluconate, which usually results in rapid recovery.
 
Hypomagnesemic Tetany (Grass or Winter Tetany)
Tetany is caused by blood magnesium falling below 0.65 mmol/L.
Often occurs along with hypocalcaemia (low blood Ca).
Lactating cows require magnesium levels of at least 0.2% of diet dry matter.
Symptoms
  • Nervousness
  • Stiff gait
  • Staggering
  • Muscle tremors
  • Convulsions
  • Paralysis and potential death
Treatment
  • Calcium–magnesium injection
  • Subcutaneous magnesium sulfate
  • Supplementing diets with magnesium oxide
 
Pregnancy Toxaemia
Occurs when energy demands of the fetus exceed the dam’s available energy during the third trimester.
Risk Factors
  • Large fetus or twins
  • Low-energy or low-protein feeds
  • Rapid weight loss
  • Poor body condition entering late gestation
Symptoms and Management
Affected cows require high-energy drenches or injections and often calving induction. Prognosis is typically poor. Diagnosis in one cow often signals herd-wide energy deficiency.
 
Key Minerals Involved in Tetany and Milk Fever
Calcium
Bone structure, muscle contraction, nerve transmission, blood clotting.
Phosphorus
Energy metabolism, bone structure.
Potassium
Electrolyte balance, nerve and muscle function.
Magnesium
Nerve transmission, enzyme activity, energy production.
 
Understanding Potassium, Tetany Ratios, and DCAB
Role of Potassium (K)
Potassium is the body’s third most abundant mineral and a major electrolyte. Potassium plays an important role in transport of oxygen and carbon dioxide in the blood and maintaining nerve and muscle activity. High forage potassium—common under manure-heavy soils, acidic soils, or drought-stressed crops—can interfere with calcium and magnesium absorption.
Normal forage K ranges: 1.5–1.9% DM.
Toxic or problematic levels: > 3% DM.
Gestating cows need 0.6% K, lactating cows 0.7% K.
 
Tetany Ratio: K/(Ca + Mg)
Research found:
  • < 2.2: Very low tetany risk (~0.7%)
  • > 2.2: Higher tetany risk (~6.7%)
Tetany ratios must be calculated in milliequivalents (mEq/kg DM) because electrical charge—not mineral weight—drives acid–base balance.
 
 
Dietary Cation–Anion Balance (DCAB)
DCAB measures the balance of positively charged ions (Na, K) and negatively charged ions (Cl, S).
A negative DCAB pre-calving enhances calcium mobilization, reducing milk fever risk.
For beef cows, a DCAB of below +150 to +200 mEq/kg DM is considered acceptable.
DCABs above 300 mEq/kg DM have been associated with metabolic disorders, including downer cows.
 
Field Example:
More than 50 herds experienced severe downer cow cases in late pregnancy or early lactation. Notable findings:
  • Forages had high K (1.5–3.26%) but tetany ratios were acceptable (<2.2).
  • DCAB values were extremely high (316–518).
  • Bloodwork showed:
    • Normal to high magnesium
    • Low calcium and phosphorus
    • Mixed clinical signs (milk fever + tetany-like symptoms)
Conclusion:
High potassium likely contributed to problems not via tetany ratio, but through elevated DCAB, impairing mineral metabolism.
 
Prevention and Management Strategies
Mineral Supplementation
  • Provide 40 g magnesium oxide (unpalatable and must be mixed with grain) and 80 g limestone per cow daily.
  • Higher supplementation may be needed for high-K rations.
  • Magnesium oxide is unpalatable—mix with grain or pellets.
  • Cereal based rations also require supplementation of salt and trace minerals and vitamins
Feeding and Testing
  • Ensure feed tests include Ca, P, K, Mg.
  • Use wet chemistry for accuracy (NIRS is less reliable for minerals).
  • Assess water mineral content.
  • Avoid relying solely on free-choice mineral—monitor intakes.
  • Force-feed 1–2 oz magnesium oxide when problems are suspected.
Ration Formulation Guidelines
  • Be cautious when forage K exceeds 2.5% DM.
  • Use low-K forages in the 6–8 weeks pre-calving.
  • Maintain Ca:P ratio between 2:1 and 7:1.
  • Aim for tetany ratio ≤ 2.2 (Cowbytes), but evaluate using mEq for accuracy.
  • Keep DCAB as low as possible, ideally < +150–200 mEq/kg DM.
  • Supplement salt, trace minerals, vitamins, and protein (especially with straw-based diets).